The present invention relates to the hydrotreatment of pyrolysis gasoline, and more particularly to processes and equipment for carrying out such hydrotreatment at reduced cost and with increased efficiency.
Pyrolysis gasoline (also referred to as “pygas”) is a liquid by-product of the steam cracking process to make ethylene and propylene. Pyrolysis gasoline is a highly unsaturated hydrocarbon mixture (carbon range of about C5–C14) that is rich in dienes, olefin and aromatics, especially benzene. In addition, pyrolysis gasoline includes undesirable heteroatom-containing hydrocarbons, such as sulfur- and nitrogen-containing compounds. To allow for its use as a gasoline blendstock, pyrolysis gasoline must be at least partially hydrogenated or hydrotreated to reduce the levels of unsaturation and heteroatom-containing hydrocarbons. Left untreated, pyrolysis gasoline typically degrades to form gums and varnishes in fuel systems.
Pyrolysis gasoline is hydrotreated today in conventional fixed bed reactors using pellet catalysts. Generally, trickle flow operation is employed. One problem with this type of operation is that incomplete wetting of the bed by liquid reactants and channeling of the gas flow through preferential flow passages may occur which reduces bed utilization. In addition, bed pressure drop is limited by the catalyst particle size and in many cases it would be desirable to reduce the catalyst particle size if the pressure drop limitation could be overcome. Additionally, conventional fixed bed reactors are prone to runaway hot spots, posing numerous health, safety, and environmental issues. Hot spots occur when catalyst is insufficiently wetted by liquid feed, and unsaturated hydrocarbons in the gas phase continue to react and release heat. Due to the exponential dependence of reaction rate on temperature, the hot spots can start to run away, leading to high temperatures and possibly even explosions.
Hydrotreating processes have been studied using alternative catalysts. For example, Smits et al., Chemical Engineering Science, 1996 (51), 3019–3025, describe a method for hydrotreating a mixture of styrene and 1-octene in toluene over a monolithic catalyst. The mixtures used by Smits et al. in their modeling experiments, however, differ significantly from actual pyrolysis gasoline compositions. For example the model system studied by Smits et al. contained only three different compounds while actual pygas contains at least 130 different compounds, and recent data indicate that the hydrogenation reaction rate of styrene in a pygas matrix is as much as 15–20 times slower than the same rate in an ethylbenzene or toluene matrix. Thus the Smits findings have little, if any, applicability to the hydrotreatment of actual pyrolysis gasoline compositions on an industrial scale.
In view of the above-described drawbacks and inefficiencies associated with conventional pyrolysis gasoline hydrotreatment as it is currently practiced, there is a clear need for continued improvement in many aspects of the process. There is a need to improve the reactors and the pyrolysis gasoline hydrotreatment process to reduce reactor size and cost, to more effectively use the hydrogen reactant, to more effectively use electricity and water, and to improve the overall bed utilization by employing better hydrodynamics. The processes described hereinafter help fulfill these and other needs.